For security detection technologies applied at sites such as customs ports, harbors and the like, radioactive substance detection technology is important one among them. Detection of the radioactive substance mainly involves detection of λ rays and neutron rays. In traditional neutron ray-based detection technologies, it is a conventional scheme to use 3He proportional counters and polyethylene moderators for fast-neutron detection. However, due to problems such as worldwide insufficient supply of 3He and high prices, practicability of the 3He proportional counters in this scheme continuously falls, so people have been seeking for an alternative solution of 3He.
The reaction type for neutron detection is a nuclear reaction, and the participators of the nuclear reaction are neutrons and atomic nucleus. A high neutron absorbing section and emergent property of charged particles as required in a high detection efficiency cause the available types of nuclides (corresponding to atomic nucleus) very limited. Generally speaking, 10B, 6Li, 155, 157Gd are the only selectable nuclides, and 10B is a preferred nuclide therein. At present, a cooperation group has been established internationally to focus on study of 10B-based neutron detectors.
In the Chinese patent application No. 201110446162.4 filed by Applicant on Dec. 28, 2011, there is provided a fast-neutron detector based on 10B as neuron reaction nuclides. The preferred embodiment of this patent application uses plastic scintillators wrapped by a boron-coated aluminum foil to achieve neutron detection, which basic principle is as follows: fast-neutrons recoil in the plastic scintillators and deliver energy to recoiled protons (or other atomic nucleus, e.g., 12C), and the protons produces ionizing light emission in the plastic scintillators to form a first signal; at the same time, the neutrons lose their energy and are moderated as thermal neutrons until they are absorbed by a boron layer adhered on the surface of the aluminum foil. 10B in the boron layer and thermal neutrons are subjected to the following reaction:
                              n          +                                                   10                        ⁢            B                          →                  {                                                                                                                                       7                                        ⁢                    Li                                    +                  α                  +                                      2.79                    ⁢                                                                                  ⁢                                          MeV                      ⁡                                              (                                                  6.1                          ⁢                          %                                                )                                                                                                                                                                                      Li                                        *                                                                7                                    +                  α                  +                                      2.31                    ⁢                                                                                  ⁢                                          MeV                      ⁡                                              (                                                  93.9                          ⁢                          %                                                )                                                                                                                                                    (        1        )            
Products of this reaction are particles 7Li and α, which both carry kinetic energy of several MeV and have opposite direction with respect to each other. One of 7Li and α penetrates the boron layer into the plastic scintillators to produce ionizing light emission, thereby forming a second signal. There is a time difference between the first signal and the second signal. The magnitude of the time difference is uncertain, but approximates an exponential distribution, and a typical average value is generally tens of microseconds, but specific average values depend on structural parameters (size of the plastic scintillators and thickness of the coated boron) of the detector.
The technical solution provided in the above-mentioned Chinese patent application has its own unique advantages, but this technology requires to plate boron on a substrate material such as aluminum foil to form a boron-containing coating film, and the output rate per unit time of this process is relatively low. Besides, on account of existence of the substrate material such as aluminum foil, only one of the two opposite charged particles α and 7Li produced in the reaction formula (1) can enter the plastic scintillator unit, which causes the resultant signal amplitude reduced, and does not facilitate improvement of the amplitude of the “second signal”, nor facilitates improvement of coincidence property.